1. Field of the Invention
This invention relates to sorting apparatus, and in particular, to sorting apparatus having a nulling arrangement operable to correct for amplifier d.c. offset and temperature drift while a sort is being conducted. The invention is also applicable to electrical circuitry wherein the quiescent state of the circuit lies outside the normal range of signal values generated therein.
2. Description of the Prior Art
It is well-known that electrical components are susceptible to voltage or current fluctuations which mask the true value of electrical signals output therefrom. These deviations are often the product of the operating environment in which the components are placed. For example, operation in elevated-temperature geographical regions and/or operating environments may impart a signal deviation known as temperature drift. Further, mechanical vibration or electrical noise may impart d.c. offset currents or voltages which distort the true component signal output. As a result it is often necessary to correct, or "null", various electrical components to accomodate and correct for d.c. offsets and temperature drifts.
Sorting apparatus for sorting a random stream of articles, such as that disclosed and claimed in the co-pending Application of J. D. P. Jones, E. H. Codding, and M. A. Smither, Ser. No. 704,651, filed July 12, 1976, now U.S. Pat. No. 4,134,498, the co-pending application of J. F. Lockett, Ser. No. 704,652, filed July 12, 1976, now U.S. Pat. No. 4,088,277, and the co-pending application of J. F. Lockett, Ser. No. 704,697, filed July 12, 1976, all assigned to the assignee of the present invention, utilize analog electronic circuit elements which are susceptible to offsets and drifts. The offsets and drifts are believed to be attributable to operating vibration and temperature. In addition, sorting apparatus such as disclosed and claimed in the last-cited co-pending applications are utilized in rugged, in-field environments, where dust, dirt, or humidity may affect the components. Further, spattering of the viewed elements with juices from the articles being sorted as well as glare and extraneous light entering the system also deleteriously affect the integrity of the sort by providing spurious readings from the electronic circuitry.
Since the apparatus disclosed and claimed in the referenced applications accept or reject articles based upon a ratio of two electrical signals representative of light reflected at two predetermined wavelengths, the necessity of assuring the integrity of the signal levels is apparent. Furthermore, various electronic circuit arrangements are associated with the basic sorter configuration which operate to provide other important classification functions. For example, the foreign object and dirt reject arrangement disclosed and claimed in the co-pending application of J. F. Lockett, Ser. No. 829,485, filed Aug. 31, 1977, and assigned to the assignee of the present invention, utilizes the dwell time of a selected signal within a predetermined signal "window" as an indication of the physical characteristic of that article. Thus, if the analog electronic elements associated with the sorting apparatus introduced a spurious reading, it is possible that a foreign object or a dirt clod would generate a signal which would not fall within the defined window. Thus, that foreign object would avoid detection and elimination from the article stream.
Several prior patents have addressed the problem of correcting d.c. offset and temperature drift in electronic components. For example U.S. Pat. No. 3,899,415, issued to E. H. Codding et al. and assigned to the assignee of the present invention, discloses and claims a nulling arrangement adapted to minimize the effects of d.c. offset and temperature drift in a sorting apparatus. However, this patent is typical of prior art arrangements which, although admirably eliminating the effect of such deviations, are usually operable only during a halt in the sorting operation. It would, therefore, be advantageous to provide a nulling arrangement operable while a sort is in progress to correct for the effects of d.c. offset and temperature drifts caused by mechanical vibration, temperature, dirt, dust, foreign material, extraneous glare, or any other reason without the necessity of halting sorting operations.